[SPOILERS] Everything you need to know about the Iron Bank of Braavos, which will be important on 'Game of Thrones' next Sunday

Warning: Spoilers for “Game of Thrones” season seven, episode three, “The Queen’s Justice.” If you aren’t caught up on the series, read at your own risk.
Sunday’s episode of “Game of Thrones” ended with an unexpected turn. The Lannister army, led by Jaime, abandoned its home of Casterly Rock and took Highgarden, while the Unsullied fought a small contingent of Lannisters.

In taking Highgarden, the Tyrell stronghold, Cersei manages to completely erase the Tyrell bloodline from existence.

While it might seem like an act of revenge on Olenna and a clever battle strategy at first, it’s a lot more than that if you think about what happened earlier in the episode. Tycho Nestoris, a representative from the Iron Bank of Braavos, shows up in King’s Landing to discuss the crown’s massive debts with Cersei. And it looks like Cersei’s plan to take Highgarden has something to do with the Iron Bank.

But what exactly is the Iron Bank of Braavos, and how influential is their power in the events to come? Find out below:

What is the Iron Bank of Braavos?

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The Iron Bank is the main bank of Braavos, and one of the wealthiest banks in the Known World. It often lends money to foreigners, including Lords of the Seven Kingdoms and the King of the Seven Kingdoms. All the Free Cities have banks, but the Iron Bank is richer and more powerful than all of them combined, and is very strict about debts and agreements.

A common saying among the Braavosi is, 'The Iron Bank will have its due.'

Have we seen it before?

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Yes.

In season four, Davos comes up with a clever plan to get a loan from the Iron Bank of Braavos. Stannis and Davos sail to Braavos and meet with Tycho Nestoris and two of his colleagues. Davos wins the loan by explaining that Stannis is the best investment to make if the bank wants the crown's debt paid back -- because it doesn't look like the Lannisters are long for this world. Stannis uses the money to re-supply his troops and head to Castle Black, just in time to save them from the wildling attack.

And remember when Mace Tyrell sings his beautiful song in the free city of Braavos in season five? Cersei sent him there accompanied by Meryn Trant to negotiate with the Iron Bank, but it's actually a bribe, which Tycho declines.

What does the taking of Highgarden have to do with all of this?

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Pretty much everything, actually.

In season four, Tywin Lannister tells Cersei that one of the reasons he agreed to ally with House Tyrell was its wealth -- it could help pay the bank back.

In the same scene, Tywin reveals that the mines of Casterly Rock are barren and have been for years. This little nugget of important information helps Cersei realise that Casterly Rock is of no value to her, despite being her home.

So without any money in King's Landing, Casterly Rock, or from any of her allies -- Euron Greyjoy has a lot of ships, leather and eyeliner, but definitely not gold -- she has Jaime and his army literally follow the money to Highgarden.

At this point, it's safe to assume that the Lannister army in Highgarden will search the stronghold for gold, and use it to pay the Iron Bank.

Why is the Iron Bank so important?

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In season four, Tywin Lannister tells Cersei about the importance of the Iron Bank, but in typical Cersei fashion, she doesn't seem to care. Tywin says:

'One stone crumbles and another takes its place and the temple holds its form for a thousand years or more. And that's what the Iron Bank is, a temple. We all live in its shadow and almost none of us know it. You can't run from them, you can't cheat them, you can't sway them with excuses. If you owe them money and you don't want to crumble yourself, you pay it back.'

And in season three when Tyrion Lannister discovers the debt Littlefinger created, he says: 'If we fail to repay these loans, the bank will fund our enemies. One way or another, they always get their gold back.'

The Iron Bank doesn't take debt lightly, and it doesn't forget.

Cersei ignored the crown's debts to the Iron Bank for years, so it's a little suspicious that it suddenly trusts her.

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Cersei's meeting with the Iron Bank in 'The Queen's Justice' goes unusually well for her, considering she's ignored the debt for years. In her meeting with Tycho Nestoris, she emphasises that a Lannister always pays his (or her!) debts. She walks away confident, but she shouldn't be: In season four, Olenna Tyrell reminds Tywin Lannister that just like a Lannister always pays his debts, 'the Iron Bank will have its due.'

And in season five, Cersei sends Mace Tyrell to Braavos on behalf of King Tommen to bribe the Iron Bank. This was probably pretty insulting to the Iron Bank. While Stannis Baratheon, a king, comes himself, Cersei sends one of the lowest ranking members of the King's Council and tries to get out of the debt easily.

Cersei definitely has a plan to pay the Iron Bank back with money from Highgarden. But the question right now is what the bank will actually do with it.

How can the Iron Bank affect the characters and the wars to come?

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In a major way.

The Iron Bank could prove its serious to Cersei after her years of not taking them seriously, by using the money she pays back to them to support her enemies: Daenerys Targaryen and/or Jon Snow.

In her meeting with Tycho in 'The Queen's Justice,' Cersei implies that the Iron Bank probably lost money as a result of Daenerys freeing the slaves, so it might not be interested in funding her claim to the Iron Throne. Tycho neither confirms nor denies this, but with Braavos being a free city, it's possible that the Iron Bank never invested in the slave trade at all, and would proudly support Daenerys.